Some people find Sarna Lotion helpful.
I would try the Original formula first. Often it works well.
The other for Sensitive is a back up if Original fails for you.
Dilantin is listed as a potential cause of peripheral neuropathy.
If this is the case for you, the itching is not so much allergic but a nerve response to the drug. Itching may precede actual pain signals.
Therefore antihistamine drugs may not work well for this type of itching.
If you can take aspirin, you could try AlkaSeltzer original formula. This has aspirin in it to reduce inflammation and also the bicarb fizz base helps change the acid environment where the nerve signals are generated, to a more basic level chemically. This result is not long lived however, and may only last 4-6 hrs. Hopefully once you clear the drug completely from your body, the itching may stop. If there is a medical reason you cannot take aspirin, then the AlkaSeltzer is not a viable option for you.
Some drugs damage the dorsal nerve roots along the spine. If this is your case, the itching may change to pain. But it is impossible to predict at this point. Let's hope it is temporary.
I certainly would not accept Dilantin treatments anymore if I were you.